The invention was developed in connection with providing an isolation tool which could be used ‘downhole’ in connection with the process of replacing the upper section of an upwardly oriented riser associated with an off-shore oil or gas production platform. Such platforms are prevalent in the Gulf coast region of the U.S. It will be described herein in that connection. However, it is contemplated that the tool and method may find application in other fields of use as well and is not limited to downhole applications in upwardly oriented pipes.
The word “riser” is commonly used to identify a steel pipe extending up from the sea floor to a production platform. Oil or natural gas is produced through the bore of the riser from one or more wells connected to it.
Many of these risers have been in use for decades in the Gulf coast region. As a consequence, in many cases their upper sections have become weakened and worn by wave action, corrosion and other damaging actions. The well operators pay close attention to this deterioration and will initiate replacement of the upper section of the riser when appropriate.
Typically the prior art replacement operation has involved:                closing the valves of the sea floor wellhead(s) to terminate production into the base of the riser;        setting a packer at depth in the riser bore (for example, this might take place at a depth of up to 40′ below the platform), so as to provide a barrier sealing off or isolating the upper section of the riser bore from its lower section;        severing the riser wall at a point above the packer with a mechanical cutting tool;        removing the severed upper section of the riser and lowering a replacement pipe section into its place; and        welding the new riser section end to end to the old section.        
It follows that one needs to isolate the pipe wall area, where welding is to occur, from the flammable gas which may still be present in the bore of the riser.
Heretofore there have been various downhole tools employed to so control the riser bore. In one case, a pig-type device or packer is displaced to the appropriate depth in the bore and then actuated from ground surface by means of an electronic signal. Upon actuation, the packer mechanically expands its seal element radially into sealing engagement with the pipe wall and anchors itself to the wall, thereby providing a barrier to gas movement along the inner surface of the upper pipe segment.
These prior art tools are closely held and are not publically disclosed in detail in the literature, to our knowledge. However, we understand that they are characterized by several shortcomings, including that:                the tools are complex and their use entails lengthy set-up times;        the tools are not adapted to monitor and provide a real time indication of the on-going competence or possible leakage of the packer seal element; and        rental of the known tools is expensive.        
It is therefore one object of the present invention to provide an isolation tool for use downhole that can be comparatively quickly installed and deployed at the desired depth in a riser bore.
It is another object to provide a tool adapted to provide a real time indication of seal competency or leakage.
And it is another object to provide a tool which, due to the simplicity of its structure and operation, can be supplied at less cost than has been common in the past.
If the tool is to be used at an off-shore platform, it is desirable that it be adapted to be insertable into the riser inlet within the space available. While these platforms are huge structures, the space available to feed an isolation tool into the open upper end of the riser bore is commonly very limited. Typically there might be only about 2½-6′ of “head room” available at the riser inlet. It therefore is a preferred object of the invention to provide a tool which is adapted to be fed into the pipe bore in spite of the usual space limitations at the inlet.
If the tool is to be used in the context of an off-shore platform riser, it is desirable that it remain cohesive in use. Dropping a steel part down the riser bore is unacceptable to the well operator. It therefore is a preferred object to provide a tool whose components are well tied together to maintain its cohesiveness while still allowing tool components to move to a limited extent in the course of ‘setting’ the tool.
If the tool is to be used in close proximity to a welding operation, it is desirable to avoid heat damage of the elastomer seals. It therefore is another preferred object to provide a tool adapted to protect the seals against deterioration from heat.